This class ensures that subclasses have access to the
MessageDrivenContext provided by the EJB container, and implement
a no-arg ejbCreate() method as required by the EJB
specification. This ejbCreate() method loads a BeanFactory,
before invoking the onEjbCreate() method, which is
supposed to contain subclass-specific initialization.

NB: We cannot use final methods to implement EJB API methods,
as this violates the EJB specification. However, there should be
no need to override the setMessageDrivenContext or
ejbCreate() methods.